Field of Endeavor
The present invention relates to energy and more particularly to electrochemical energy storage.
State of Technology
Electrochemical energy storage is required for grid storage, wireless communications, portable computing, and will be essential for the realization of future fleets of electric and hybrid electric vehicles, which are now believed to be an essential part of the worlds strategy for reducing our dependence on oil, and minimizing the impact of gaseous emissions of CO2 on global warming. In looking at those possible materials that can be used for anodes in electrochemical energy conversion and storage systems, hydrogen and lithium have the highest specific capacities (Ah/kg). Hydrogen is of course used to power fuel cells, while lithium is used in advanced rechargeable batteries.
Most state of the art energy storage systems use lithium ion battery chemistry, with graphite anodes that intercalate lithium upon charging, mixed transition metal oxide cathodes that intercalate lithium during discharge, a micro-porous polyethylene electrode separator, and electrolyte formed from a dielectric mixed solvent composed of organic carbonates and high-mobility lithium salts. The movement of the lithium ions between the intercalation anodes and cathodes during charge and discharge is known as the “rocking chair” mechanism.
Cells with liquid electrolytes are usually contained in cylindrical or prismatic metal cans, with stack pressure maintained by the walls of the can, while cells with polymer gel electrolytes are usually contained in soft-side aluminum-laminate packages, with stack pressure achieved through thermal lamination of the electrodes and separators, thereby forming a monolithic structure.
The active graphite or transition metal oxide materials used in the electrodes exist as fine powders, coated onto thin metal foils of copper and aluminum, respectively, and held in place by a PVDF binder. Both natural and manmade graphite such as MCMB have been used for the anodes, while LixCoO2, LiNiO2, LiMn2O4, mixed transition metal oxides with cobalt, nickel and manganese, and iron-phosphates are common choices for the cathode.
Over the past decade, these systems have attained outstanding specific energy and energy density, exceptional cycle life and rate capabilities that enable them to now be considered for both vehicular and power tool applications, in addition to their early applications in wireless communications and portable computing. The best commercially available, polymer-gel lithium ion battery now has a specific energy of 180 to 200 Wh/kg, an energy density of 360 to 400 Wh/L, and a reasonably good rate capability, allowing discharge at C/2 or better.
Both liquid prismatic and polymer gel cells have been incorporated into large high-capacity power packs and used to power the mobile electric vehicles. Such high capacity systems have state-of-the-art computerized charge and discharge control, which includes graphical user interfaces, sensing for monitoring the health of individual cells, and charge balancing networks.
Such lithium ion batteries, which rely on the rocking chair mechanism, are generally believed to be safer than those where lithium exists in the reduced metallic state. However, the use of flammable liquid-phase and two-phase polymer gel electrolytes, coupled with a high energy density, a relatively delicate 20-micron thick polymeric separator, and the possibility of lithium plating and dendrite formation due to non-uniform stack pressure and electrode misalignment has led to safety problems with these energy storage systems. The possibility of such an event occurring on commercial airliners, where many passengers carry laptop computers and cell phones with such batteries, is especially disconcerting. These events have occurred on much larger scale, and have caused industry-wide concern in the continued use of this important technology.
Adequate and intelligent thermal management in these cells is essential. High rates of charge or discharge drives the temperature upward due to resistive heating of the electrolyte. When the core temperature of these cells exceeds approximately 150° F., the systems frequently become unstable, with the possible initiation of autocatalytic reactions, which can lead to thermal runaway and catastrophic results. Disproportionation of the transition metal oxides can liberate sufficient oxygen to support oxidation of the organic carbonate solvents used in the liquid or polymer-gel electrolytes. It is now recognized that while conventional systems provide high energy density, their safety remains problematic.
The treatise, Introduction to Nanotechnology, by Charles P. Poole, Jr., and Frank J. Owens. John Wiley &. Sons, 2003, states: “Nanotechnology is based on the recognition that particles less than the size of 100 nanometers (a nanometer is a billionth of a meter) impart to nanostructures built from them new properties and behavior. This happens because particles which are smaller than the characteristic lengths associated with particular phenomena often display new chemistry and physics, leading to new behavior which depends on the size. So, for example, the electronic structure, conductivity, reactivity, melting temperature, and mechanical properties have all been observed to change when particles become smaller than a critical size.”